I am currently working on creating an experiment similar to the icub visual tracking experiment. For this I want to import a custom blender model (of a triangle) without textures into the environment and manipulate it during the experiment using transfer functions.

I tried to mirror the way other custom models are integrated into the room (for example vr_lamp) however the simulation seems to either ignore my object or say that "assets have not been found."

the model is currently saved in a folder $HBP/Models/triangle/ with a model.config model.sdf file as well as a "meshes" folder which contains a .dae file.

You need to add your model to the Models/_rpmbuild/models.txt file and rebuild the platform to create the appropriate links to the new models. You can use the fast build: $HBP/user-scripts/update_nrp build (without the "all" parameter).